Electronic pens have been developed as a complement to keyboards and mice for controlling computer-based systems and inputting information to them. Such a pen is associated with an internal and/or external position detector which electronically records the pattern of movements of the pen over a base and generates an electronic copy thereof, below referred to as electronic handwriting.
The position detector can be based on, for instance, recording of a position-coding pattern on the base, mechanical detection of movements, use of accelerometers and/or gyroscopes, triangulation of electromagnetic or acoustic signals, pressure detection in the base, recording of changes in an electromagnetic field, or a combination of such techniques.
In many electronic pens, the position detector is activated based on an output signal from a pressure sensor which is mechanically connected to the pen stem to detect when the pen is made to abut against a base.
When reproducing electronic handwriting on a display or a base, readability is frequently dependent on the fact that the reproduction also reflects dynamic parameters of the handwriting, such as the force of application of the pen stem during writing. The above pressure sensor should thus be designed to emit an output signal that is representative of the force of application.
An electronic pen of this type is known from e.g. U.S. Pat. No. 6,252,182. The pen has a pen stem whose inner end is connected with a control block which is slidingly received in a cylindrical cavity in the body of the pen. Opposite to the control block, there is arranged in the cavity an electrode plate with an inner circular conductor and an outer helical conductor. A helical spring is arranged round the control block in the cavity for biasing the same away from the electrode element into abutment against a stop lug. On its side facing the electrode elements the control block has a pointed application element of conductive rubber. In an unloaded initial position, the application element with its point is received in a central guide hole in the electrode element and is in electrical contact with the circular conductor. When exerting a load on the pen stem, the application element is gradually urged against the electrode plate so that the contact surface between them increases, whereby the electrical resistance over the electrode plate gradually changes. Based on the voltage across the electrode plate, a measure of the pen stem's application force on a base is thus obtained.
A pen of the above construction, however, suffers from several drawbacks.
The sensor arrangement of the pen is relatively complicated to assemble, which makes production as well as repair and maintenance difficult and expensive.
Moreover the sensor arrangement of the pen will probably have a limited life by the point of the application element being quite heavily worn when cooperating with the guide hole in the electrode plate,
A further drawback is that the characteristics of the sensor arrangement, i.e. its functional relationship between application force and output signal, can only be modified within narrow limits and then by way of radical construction changes of the sensor arrangement.
Further examples of pressure-sensitive electronic pens are given in U.S. Pat. No. 6,104,388, U.S. Pat. No. 5,895,895, U.S. Pat. No. 5,571,997 and U.S. Pat. No. 5,548,092.